1. Field of the Invention
The present invention relates to a lightweight bearing, and to a wave gear drive in which a rigid, circular internal gear and a flexible, circular external gear are coupled by the lightweight bearing so that the gears can rotate relative to each other.
2. Description of the Prior Art
In JP-A 09-250608, now U.S. Pat. No. 6,682,220, this applicant proposed a compact wave gear drive unit that uses a cross-roller bearing. The compact wave gear drive unit of this disclosure included first and second end-plates disposed at each end in the unit's axial direction, a top-hat-shaped wave gear drive incorporated between the end-plates, and a cross-roller bearing that rotatably supports a top-hat-shaped flexible external gear and rigid internal gear constituting the wave gear drive, a drive housing being comprised by an outer ring of the cross-roller bearing and the first and second end-plates. In JP-A 09-303496, this applicant also proposed a wave gear drive unit having a similar structure, that included a cup-shaped wave gear drive.
Such wave gear drive units are compact enough to be used as reduction gear mechanisms of actuators incorporated in robotic arms and other such applications in which installation space is limited. Generally, it is necessary for the constituent components of such mechanisms to be light in weight. In a wave gear drive unit, the cross-roller bearing and rigid internal gear in particular are heavy, so if the weight of these parts could be reduced, it would make it possible to reduce the weight of the wave gear drive unit.
However, the raceway surfaces of the cross-roller bearing and the teeth of the rigid internal gear have to be durable and wear-resistant, which has necessitated the use of heavy iron-based materials. Moreover, the bearing rings of the cross-roller bearing and rigid internal gear are affixed to other members such as the unit housing, so if such members are made of a lightweight material such as aluminum alloy, the members are not able to withstand the seating pressure of the fixing bolts, leading to seating faces that are defective or strained and the like, thereby degrading the fixing force. For these reasons, it has been difficult to reduce the weight of cross-roller bearings and wave gear drive units.
A main object of the present invention is to provide a lightweight cross-roller bearing and wave gear drive in which the light weight is achieved without harming the performance of the bearing raceway surfaces or rigid internal gear teeth while at the same time ensuring the seating-face strength of the fastening surfaces of the bearing rings and rigid internal gear of the wave gear drive.